This invention relates to a mobile radio terminal, such as a mobile telephone, adaptable to at least two kinds of mobile telephone systems, such as a GSM (Global System for Mobile communication) (2G) and a third-generation mobile telephone system (3G) and, in particular, to a cell search method for at least two kinds of mobile telephone systems.
When a mobile telephone is located outside a service zone of a mobile telephone system or at a time instant of power on, the mobile telephone is in a no-service mode in which location registration is not yet carried out between the mobile telephone and a base station and thus transmission and reception are unavailable. Therefore, the mobile telephone has a cell search function of periodically searching the base station for the purpose of location registration to the base station.
A cell search cycle has a large influence upon current consumption and user friendliness. Specifically, in order to reduce the current consumption, it is advantageous to lengthen the cell search cycle so that cell search is not so frequently carried out. However, if the cell search cycle is too long, the location registration may not be carried out for a long period of time even after entry into a cell zone. This is because the entry into the cell zone can not be recognized until the cell search is carried out. Thus, user friendliness is not good. On the other hand, if the cell search cycle is shortened, the location registration is carried out immediately after entry into the cell zone. Thus, the user friendliness is improved. However, power consumption required for the cell search is increased.
In view of the above, use has been made of a method of adjusting the cell search cycle depending upon the circumstance. For example, the cell search is carried out in a short cycle for a predetermined period of time after the mobile telephone moves out of the service zone and, in case where no cell (i.e., no base station) could be found for the predetermined period of time, the cell search cycle is lengthened stepwise.
For example, Japanese Patent Application Publication (JP-A) No. 2001-145162 proposes a method of the type mentioned above. Specifically, when the mobile telephone moves out of the service zone, the cell search cycle is lengthened in correspondence to a time duration for which the mobile telephone stays outside the service zone (i.e., the mobile telephone is kept in a no-service mode). When a user executes a predetermined operation (for example, presses down a search switch) outside the service zone, the cell search is immediately carried out. If the entry into the cell zone is detected as a result of the cell search, a service mode is immediately started and the location registration is carried out. If the mobile telephone is still located outside the service zone, the no-service mode is displayed and the cell search in a long cycle is carried out again.
Also in a mobile telephone (dual system mobile radio terminal) adaptable to two kinds of mobile telephone systems or networks (hereinafter referred to as mobile telephone systems) represented by 2G and 3G systems, a long cell search cycle is set for each of these systems in case where the mobile telephone is located outside service zones of both of the systems. For example, in a 2G standby state (2G_Idle period) where transmission and reception are available in the 2G system after the mobile telephone enters the service zone of the 2G system and location registration to the 2G system is completed, a cell search cycle in the 3G system is lengthened or the cell search itself is not carried out until a cell search switch is pressed.
Therefore, if the mobile telephone enters the service zone of the 3G system during the 2G_Idle period, the entry into the service zone of the 3G system can not immediately be detected unless the end of the cell search cycle for the 3G system is approaching or unless the cell search switch is pressed. Thus, transmission is often carried out using the 2G system for which the location registration is already completed. This results in a problem that the chances of utilizing the merits of the 3G system (speech quality, communication rate, and the like) are reduced.